1. Field of the Invention
The present invention relates generally to acoustic sensing systems, and more specifically relates to a system for sensing acoustic waves comprising an acoustic sensor array.
2. Description of the Related Art
Typically, to obtain oil, a well or hole is dug by drilling and removing earth from the ground to form a shaft known as a "borehole," which extends to the bottom of the well. Generally, a large metal pipe or casing will be inserted into the borehole. Smaller pipes, known as production tubes, are inserted into the casing. These production tubes allow access to the bottom of the well. For example, oil may be drawn from the well through the production tubing.
Ultimately, the well will appear to go dry. Despite the apparent lack of oil within the well, vast supplies of oil are often trapped in pockets in the earth nearby the well. These pockets, however, are generally inaccessible to the drilled well. To locate such pockets, known in the art as "in-place" reserves, geologists conduct surveys of swaths of earth surrounding the wells. Geologists employ techniques like cross-well tomography in which acoustic waves are transmitted through a volume of earth to characterize properties, such as density, in that volume. Knowledge of the density of the earth helps determine the presence or absence of oil in the region of the earth being characterized.
To survey the transmission characteristics of a region of the earth, an acoustic wave source can be used to generate acoustic waves, i.e., sound, while an array of acoustic sensors detects these acoustic waves. Generally, each of the sensors in the array will be situated at a different location. The acoustic waves emitted from the acoustic source are thus sampled at a plurality of points which typically make up a line. By changing the location of the acoustic source, the location of the sensor array, or both, the transmission characteristics of a volume of earth may be measured. In this manner, a three-dimensional map of the density throughout a region of earth can be produced.
Although some prior art techniques rely on acoustic sources and/or sensor arrays situated on the surface of the earth, placing the acoustic sources and sensor arrays deep within the earth is more effective for surveying lower regions of the earth. To conduct measurements deep within the earth, a probe can be lowered into the well.
However, the frailty of conventional prior art sensors prevents prior art sensor arrays from being employed deep within a well. Conventional sensor arrays employ piezoelectric transducers (or piezos) to convert vibrations originating from the acoustic waves into electronic signals. Since a piezoelectric transducer outputs only a small signal, an electronic preamplifier must be mounted near the piezo to prevent noise from overwhelming the small transducer signal. Electronics, however, are incompatible with the harsh environmental conditions, such as high temperature and pressure, that prevail deep within the earth. Even preamplifiers designed to survive high temperature have a short lifetime and may last, for example, only for one hour under harsh conditions. Thus, the requirement for an electronic preamplifier prevents piezoelectric transducers from being employed deep within a well.
Fiber optic sensors, on the other hand, are electrically passive devices. That is, they do not require electrical components or external electrical connections. Thus they are less susceptible to the harshness associated with high temperature, high pressure environments. Furthermore, fiber optic sensors avoid the environmental problems associated with electrical components, e.g., the electromagnetic interference that arises when electrical components are placed in the presence of transmission lines. For these reasons, fiber optic sensors are sometimes used in hydrophones operating under harsh environmental conditions.
Fiber optic hydrophones can generally be classified into two categories. Hydrophones of the air backed mandrel design have a hollow, sealed cavity that deforms in response to acoustic pressure, so that strain is transferred to the fiber wrapped around the mandrel. Other, less sensitive, fiber optic hydrophone designs record the effects of pressure directly on the fiber itself, e.g., the fiber may be wrapped around a solid body. Fiber optic hydrophones with high sensitivity (i.e., air backed mandrel hydrophones) are generally limited to operating pressures of less than about 5000 pounds per square inch (psi) and temperatures of less than about 120.degree. C. Outside this range, the materials used in the mandrels of air backed mandrel hydrophones deform excessively. For example, polycarbonate plastic deforms at these temperatures, whereas metals such as aluminum buckle in elastically when subjected to high pressures. On the other hand, fiber optic hydrophones utilizing solid bodies or fiber for acoustic transduction typically have much lower sensitivities.
In addition to operating limitations on pressure and temperature, current fiber optic hydrophones are generally bulky, and may have large cross sections that do not lend themselves to use in applications where compactness is essential, e.g., in commercial petrochemical wells and boreholes. Thus, there is a need for a fiber optic hydrophone having a relatively small cross section and the ability to withstand high pressures and temperatures.
In addition to restrictions on the placement of the prior art acoustic arrays, limitations exist on the number of sensors that may be employed in prior art acoustic arrays. With a larger number of sensors more information must be processed. Limitations on the amount of information that can be processed within a reasonable amount of time restrict the number of sensors that can be used. Higher resolution maps, however, can be achieved with a larger number of sensors.
Thus, a need exists for a system for sensing acoustic waves that is rugged enough to operate in the harsh downhole environment and accommodates a large number of sensors.
Systems accommodating a large number of sensors may benefit from the use of multiplexing, in which multiple signals are communicated within a single line. One common approach, known as frequency division multiplexing (FDM), operates by modulating a carrier wave at a number of different frequencies equal to the number of signals that are to be multiplexed. When FDM is applied to a system using interferometric sensors, the multiplexed signal includes signal components not just at the modulation frequencies, but at all harmonic frequencies of the modulation frequencies as well. For such a system, the multiplexed signal may be demultiplexed through detection of the signal components at the modulation and first harmonic frequencies, provided these components do not overlap (in frequency) one another or any components at the higher harmonics. Such overlap may be prevented by selecting modulation frequencies that are sufficiently large and separated that the lowest second order harmonic component exceeds the highest first harmonic component. This leads to large bands of unused frequency between DC and the highest frequency signal component detected. However, to keep the signal processing electronics simple it is preferable to keep the maximum frequency detected as low as possible. Thus, a need exists for a method of selecting a set of FDM modulation frequencies having as low a maximum frequency as possible while maintaining fundamental and first harmonic signal components that are not overlapped by other signal components.